Optical systems can be usefully classified as being single-mode or multi-mode. In a single mode system, there is only one possible spatial pattern (i.e. the “mode”) of optical amplitude and phase. In a multi-mode system, there are two or more such possible spatial patterns of optical amplitude and phase. A free space optical system can be regarded as having an infinite number of modes, although in practice there are effectively a finite number of relevant modes. For example, the number of resolvable spots in an imaging system would be on the same order as the number of relevant modes in that system.
In principle, each mode can be accessed independently of the other modes. For example, a multi-mode fiber telecommunication system using fiber that supports 100 modes would in principle have 100 independent communication channels on that single fiber (all at the same optical wavelength).
However, the difficulties in actually providing independent access to these 100 different modes are formidable, especially because small perturbations to the fiber (which can vary in time) will cause the relative phases (incurred in transmission) of the fiber's modes to change. Thus, any approach for accessing the 100 modes in the fiber of this example would have to adapt in real time to account for these changing relative phases, which can completely alter the received intensity pattern from the fiber by constructive and destructive interference.
In fact, long haul telecommunications uses single-mode fiber in nearly all cases, in large part to avoid complexities such as those described above. Accordingly, it would be an advance in the art to provide improved handling of multi-mode wave propagation.